1. Field of the Invention
The invention relates to a call waiting notifying apparatus for notifying the generation of a call for a terminal in communication to terminal in communication.
2. Description of the Related Background Art
Hitherto, in a communication through an analog public network, if a new reception request has been generated in calling, such a fact is notified from the analog public network as a calling sound mixed into a call voice. Therefore, in a private branch exchange in which a plurality of analog public lines are enclosed and a plurality of analog extension terminals are connected, if a call to one of the extension terminals has been received in communication through the analog public line, a call waiting is notified to such an extension terminal by a calling sound mixed in the call voice. That is, the private branch exchange itself doesn't execute any process regarding the operations during the communication and reception.
On the other hand, in the communication through a digital public network, in the case where a new reception request has been generated in calling, a call waiting notification is received from the public network through a control channel and the occurrence of the reception can be known. However, in the private branch exchange in which a plurality of digital public lines are enclosed and a plurality of extension terminals are connected, if a call waiting has occurred in the digital line in which the extension terminal is in communication, the private branch exchange can recognize the occurrence of the call waiting, but the analog extension terminal cannot recognize the occurrence of the call waiting.
FIG. 1 is a block system diagram showing a construction of a conventional private branch exchange. In the diagram, reference numeral 1 denotes a main apparatus of a private branch exchange; 2 a communication line; 3 a line I/F (interface) unit; 4 a central control unit (CPU); 5 a highway switch; 6 a subscriber's circuit; 7 a digital/analog converting circuit (D/A converter); 8 an analog/digital converting circuit (A/D converter); 9 a data bus and an address bus; 10a and 10b channels (highways); 11 a subscriber's line; and 12a to 12c single telephones (SLT) (hereinafter, shown by common reference numeral 12).
The operation of the apparatus of FIG. 1 will now be described.
If a call reception from the line (analog line) 2 has been made, the line I/F unit 3 detects the call reception and notifies the reception to the CPU 4. The CPU 4 allows a preset one of the extension single telephones 12a to 12c to ring through the subscriber's circuit 6. When the SLT 12 is off-hooked, the subscriber's circuit 6 detects the off-hook and notifies to the CPU 4. The CPU 4 switches the highway switch 5 so as to connect a time slot allocated to the analog line to which the call reception has occurred and a time slot allocated to the off-hooked single telephone, thereby starting a communication. That is, the line 2 is connected to the SLT 12 through the line I/F unit 3, highway 10a, highway switch 5, highway 10b, subscriber's circuit 6, and subscriber's line 11.
If a call waiting request has been generated from the analog line in communication, the reception request notification directly arrives at the SLT 12 in a state in which it is mixed into a voice signal. When the user of the SLT 12 recognizes the reception request notification signal and executes a hooking operation, the subscriber's circuit 6 detects the hooking and notifies to the CPU 4. The CPU 4 notifies the transmission of the signal indicative of the hooking to the line I/F unit 3. The line I/F unit 3 sends a hooking signal to the line 2 and notifies the acceptance of the reception to the analog network.
In the digital line, generally, the data such as a voice or the like which is needed by the subscriber and the data which is necessary for exchange and connection are separated. If a reception request has been generated in calling, the reception request notification is sent to the line I/F unit 3 through a channel (hereinafter, referred to as a control channel) to transmit the data which is necessary for exchange and connection. Although the line I/F unit 3 can notify a fact of the call waiting to the CPU 4 through the data bus 9, the I/F unit 3 cannot notify a fact of the call waiting to the SLT 12 connected to the extension.
As mentioned above, in the above conventional apparatus, if a call waiting has been generated through the digital line, the private branch exchange can recognize the generation of the call waiting but the extension terminal cannot recognize the generation of the call waiting, so that the call waiting service cannot be effectively utilized.
On the other hand, it is impossible to determine from which partner a new reception has been performed, so that a significance about such a reception cannot be presumed. That is, the calling person cannot compare which one of the present call in calling and the new reception has a priority, so that he cannot help temporarily interrupting the present call in calling in order to know the content of the communication of the new reception. Moreover, there is a case where if the called party had answered the calling party of the new call, its significance is so low that there is no need to interrupt the present call in calling or a case where in spite of the fact that a communication with the calling party of the new call should have been given the priority, the new reception is abandoned in calling and the call is disconnected because the call interruption is too late.
The calling party can be discriminated by the calling party number information which is sent from an ISDN (Integrated Digital Service Network). However, if a telephone of the called party is an analog telephone, the calling party number information cannot be directly sent by a communication control procedure of the ISDN, so that the calling party cannot be identified by using the calling party number information upon call waiting.